Electrical discharges in crossed fields (EXB discharges) attract much attention due to their importance for science and technology. In science EXB discharges are important in the field of plasma physics and cosmic physics. In technology EXB discharges are used in devices for thermonuclear fusion, in vacuum technology such as in vacuum pumps, vacuum measurements, for coating work pieces using e.g. magnetron sputtering, in plasma accelerators, and as plasma emitters in ion sources.
The motion of charged particles in stationary crossed fields and quasi-stationary EXB discharges have been studied since 1921, see the article by A. W. Hull, “The effect of a uniform magnetic field on the motion of electrons between coaxial cylinders”, Phys. Rev. 18, 1921, pp. 31-57, and by H. C. Early, W. G. Dow, “Supersonic Wind at Low Pressures Produced by Arc in Magnetic Field”, Phys. Rev. 79, 1950, p. 186. Such discharges could be classified according to different parameters such as gas pressure, strength and configuration of the magnetic field used, electrode configuration etc. For the purposes herein these discharges are best classified according to the intensity or generally the behaviour of the discharge or driving current.
According to this classification using the driving current, quasi-stationary discharges in crossed fields could be divided in two classes: low intensity and high intensity current discharges. It is necessary to note that the transition current depends on many parameters, in particular on the dimensions of the apparatus used, and can vary for hundreds of amperes. Low intensity current discharges in crossed fields could be called such discharges which produce a plasma inside a magnetic configuration with a density less than 1018 m−3 and high intensity current discharges could be called such discharges which produce a plasma having a density of more than 1018 m−3, the plasma density defined as the number of particles per unit volume.
Low intensity current discharges in crossed fields are widely used in vacuum technology such as in vacuum pumps, for coating work pieces, e.g. in magnetron sputter deposition. Typical discharge devices are Penning cells and cylindrical and planar DC-magnetrons. The low driving current results in a low-density plasma, less than 1018 m−3 as indicated above.
High intensity current discharges have been mostly used for generating dense plasma for the goals of thermonuclear fusion. Typical discharge devices include Homopolar I, Ixion and F I devices. The typical plasma density is about 1018-1023 m−3.
The second important characteristic of discharges in crossed fields is the voltage drop between the electrodes.
For a low intensity driving current the rate of neutral gas ionization is low and balances the plasma losses to form an equilibrium plasma density at a low level. The electrical resistance of the anode-cathode gap is high resulting in a high anode-cathode potential drop. As soon as an opposite process becomes energetically possible a strongly enhanced ionization process should arise.
Two methods have been described for plasma ionization in systems using with discharges in crossed electric and magnetic fields. Their practical applicability depends on system dimensions and the strength of the magnetic field. The method generally accepted in systems of sufficiently large dimensions using a strong magnetic field is the so called “Rotating Plasma Approach”. This approach is based on the fact that the electric field penetrates into the plasma and that the plasma is magnetized, see B. Lehnert, “Rotating Plasmas”, Nuclear Fusion 11, 1971, pp. 485-533. Another approach is based on fact that the electric field is concentrated preferably near the cathode of the discharge. This approach is used for processes in systems using a low magnetic field and non-magnetized ions. This approach could be called e.g. “Secondary Electron Approach”, see B. S. Danilin and B. K. Sirchin, Magnetron Sputtering Systems, Moskva, Radio i Sviaz, 1982. As will be obvious from the following this invention deals with both kinds of systems and therefore both plasma approaches will be used.
Alfvén has postulated, see H. Alfvén, “On the Origin of the Solar System”, Clarendon Press, Oxford, 1954, that a strongly enhanced ionization process should arise when the mutual plasma-neutral gas velocity reaches the critical value vc, the Alfvén limit, given byvc=(2e¢i/mi)1/2 where ¢i is the ionization potential, e is the charge of the electron and mi is the ion mass.
For devices having a low sputtering rate and low plasma losses it results in an anode-cathode voltage drop limitation during the starting period of the discharge. For devices having a high sputtering rate it results in an anode-cathode voltage drop limitation during all of the discharge time. The voltage drop or critical voltage Vc is given byVc=CvcBwhere C is a constant and B is the strength of magnetic field in the discharge device. In the case of a high sputtering rate, the ionization potential ¢i of the sputtered atoms creates the metal vapor. It means that the discharge voltage has to depend on the sputtering cathode material.
This phenomenon was demonstrated both by investigation of plasma motion through a neutral gas and by experiments with planar magnetron sputtering devices, see U. V. Fahleson, “Experiments with Plasma Moving through Neutral Gas”, Physics Fluids, Vol. 4, 1961, pp. 123-127, and D. V. Mozgrin, I. K. Fetisov, and G. V. Khodachenko, “High-Current Low-Pressure Quasi-Stationary Discharge in a Magnetic Field: Experimental Research”, Plasma Physics Reports, Vol. 21, No. 5, 1995, pp. 400-409. In the latter publication the high current, low voltage discharge in a magnetron magnetic configuration is called as a “high-current diffuse regime”.
It means that the transition from a low intensity current EXB discharge to a high intensity current discharge has to be followed by a decrease of the discharge voltage. Typical anode-cathode potential drops for low intensity current, quasi-stationary discharges are in the range of about 10-0.3 kV and for high intensity current discharges in the range of about 300-10 V.
If quasi-stationary discharges are implemented in magnetron sputtering devices, in a first regime effective cathode sputtering is obtained but a low ionization rate of the sputtering gas and metal vapor. In a second regime an opposite state occurs having a low sputtering rate but a high ionization rate of the sputtering gas. Thus, it can be said that it is impossible to generate, by a separate low intensity current quasi-stationary discharge, or by a separate high intensity current discharge in crossed fields, highly ionized metal plasma fluxes.
The devices using EXB discharges can operate for a short time in the transient, i.e. the non-quasi-stationary, regime. In this regime it is possible to overcome the Alfvén limit of discharge voltage as well for high current discharges, see the article by B. Lehnert cited above. High current, high voltage non-quasi-stationary discharges occur in magnetron sputtering devices and are very important for magnetron sputtering applications because those discharges allow obtaining a fully ionized impermeable plasma in the magnetron magnetic configuration. But, as will be shown hereinafter, if transient discharges are implemented in magnetron sputtering devices by either high intensity current discharges or by low intensity current discharges it is impossible to generate highly ionized intensive metal plasma fluxes.
Metal plasma fluxes can be produced by low current quasi-stationary EXB discharges in a magnetron configuration for sputtering atoms in a moderate pressure, of e.g. 1-100 mTorr, and with a low-density plasma In this case the plasma is produced by an RF-induction coil mounted in the deposition chamber. The electron density produced in induction plasmas is about 1017-1018 m−3.
This method of coating work pieces has important implications for the filling of high-aspect-ratio trenches and vias encountered in microelectronic fabrication processes as well as in sputtering magnetic materials and modifying the properties of thin films by energetic ion deposition, see J. Hopwood and F. Qian, “Mechanisms for highly ionized magnetron sputtering”, J. Appl. Phys. 78 (12), 15 Jul. 1995, pp. 758-765.
The drawbacks of this method of metal plasma production include the complexity of the RF-ionization technique and the high pressure of the sputtering gas required for producing the low-density plasma. The high pressure of the sputtering gas is required because of the high energy consumption necessary for producing a low-density plasma.
The discharges in crossed fields could be implemented by simple techniques and within an extremely wide range of operating pressures: from 10−11 up to 102 Torr. Low-pressure magnetron discharges, up to 10−5 Torr, can be achieved because of the selfsputtering phenomenon, see for example S. Kadlec and J. Musil, “Low pressure magnetron sputtering and selfsputtering discharges”, Vacuum, Vol. 47, pp. 307-311, 1996. This method of coating work pieces has important implications for the etching of surfaces by metal ions for increasing the adhesion of deposited layers and for the filling of high-aspect-ratio trenches and vias encountered in micro-electronic fabrication.
The currents necessary for generating a dense plasma and sustaining it by high intensity current EXB discharges are large enough for cathode spots, and possibly also for anode spots, to be formed at the cold electrode surfaces. Devices having such electrodes should therefore have a natural tendency of forming spoke-shaped azimuthal plasma inhomogeneities, arc discharges, see B. A. Tozer, “Rotating Plasma”, Proc. IEEE, Vol. 112, 1965, pp. 218-228. Such conditions are strongly pronounced in all types of devices during the starting period of the discharges where a large driving current is needed for neutral gas burn-out.
Having cold electrodes and neutral-plasma phenomena in mind, the experiments on plasma spoke formation can be summarized as follows:    a. In the Homopolar III experiments it was found that during the starting period the discharge current was confined to a set of about 10 to 12 narrow radial spokes, arcs, rotating with the plasma. See W. R. Baker, A. Bratenal, A. W. De Sliva, W. B. Kunkel, Proc. 4th Int. Conf. Ionization Phenomena in Gases 2, Uppsala 1959, North-Holland Publishing Comp., Amsterdam, p. 1171, and W. B. Kunkel, W. R. Baker, A. Bratenahl, K. Halbach, “Boundary Effects in Viscous Rotating Plasmas”, Physics Fluids, Vol. 6, 1963, pp. 699-708.    b. In the Leatherhead Homopolar device having a negative polarity one or two spokes were observed to arise during the initial breakdown of the discharge. They were soon smeared out to form spirals with an increasing velocity in the outward radial direction, see P. B. Barber, M. L. Pilcher, D. A. Swift, B. A. Tozer, C. r. de la VIe conference internationale sur les phenomenes d'ionization dans le gas 2, Paris, 1963, p. 395.    c. In the Kruisvuur I device a single eccentric structure rotating around the axis with a velocity close to E/B was observed, see C. E. Rasmussen, E. P. Barbian, J. Kistemaker, “Ionization and current growth in an ExB discharge”, Plasma Physics, Vol. 11, 1969, pp. 183-195.
From the experiments mentioned above and others, arc formation is clearly seen to be connected with the starting period of the high intensity current EXB discharges in most devices.